The present invention relates to an electrolysis cell having bipolar elements for the electrolysis of, in particular, solutions of alkali salts.
Electrolysis cells having bipolar elements have been known for a long time. They have the advantage of a compact structure and thus a saving in space and facilitated electricity supply due to the fact that unit cells are connected in series.
Nevertheless, despite their obvious advantages such cells are still relatively little used. This is very largely due to the difficulties in production, particularly difficulties of a technical nature. In actual fact it is necessary to maintain a small interpolar distance. This means that large plane surfaces are arranged opposite one another and as parallel as possible, but also with a high degree of impermeability between each compartment and each element, which is very difficult to effect with solutions as corrosive as the brines treated in this type of cell.
In addition, the passage of a current and the circulation of gas and liquid occur in these cells, which means that the gases must be able to escape satisfactorily and the liquids must be able to circulate despite the small size of the anode and cathode compartments. Now, it is known that it is not easy to satisfy both the mechanical and electrical requirements at one and the same time.
Several types of solution have been proposed in order to obtain bipolar elements which are both in the same plane and allow current to pass, either by trying to separate the electrical and mechanical functions or by making the base plate of the electrode act just as a support, as in de Lachaux et al. U.S. application Ser. No,. 478,605, filed June 12, 1974 now U.S. Pat. No. 3,980,545. In that application, the current leads act as stiffeners and ensure the planarity of the anode and cathode parts with respect to the base plate.
Another serious difficulty encountered in this field of technology is the question of impermeability between the various units constituting the cell.
A solution which is often adopted is to arrange a sealing member between two consecutive rigid units. This simple solution unfortunately does not satisfy the requirements which are met in commercial practice and this is why it has recently been proposed to give the frames a particular shape, just resting against adjacent frames and having at least one recess in which at least one sealing member is arranged.
However, for such a solution to give good results it presupposes that the various bipolar elements are put in place correctly and that the clamping or securing forces are constant and uniformly distributed. This requirement follows from the very technology of the cell. Now, up to the present time the technologies of cells having bipolar elements can be classified in two large categories. One of these is the so-called filter press type. In this, various elements are secured to one another by jacks or tie-bars joining the end elements.
However, this arrangement has two serious disadvantages the first being that it is practically impossible to obtain a uniform clamping force between all the pairs of elements. Furthermore, any slight lack of planarity will have repercussions from element to element. The result is that even with sealing means which are useful in themselves, this arrangement is practically impossible to implement satisfactorily with a simple installation and at reasonable cost. The other disadvantage is due to the fact that in the event of deterioration of an element and its sealing means, it is necessary to stop operations and disassemble the cell.
Attempts have been made to improve this arrangement by no longer having a vertical assemblage, but instead having a horizontal stacking system. In this case, the removal of gases formed during electrolysis becomes more difficult.
The second category of cells has an arrangement which, by analogy, can be termed an accumulator type arrangement. In this case various elements are placed side by side in a tank. Although the positioning can then be guaranteed precisely, this is no longer the case with the clamping, and assembly, disassembly and impermeability are not completely satisfactory.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art.
It has now been found that these disadvantages can be obviated simply and unexpectedly by means of the cell, which is an object of the present invention, characterised in that the cell consists of an assemblage of units whose cohesion is ensured by assembly means from unit to unit.